Devices for measuring dimensions by pneumatic means are known that comprise a source of gas under pressure, a measuring branch connected to the source of gas under pressure and including a feed nozzle and a measuring member, and a pressure sensor associated with the measuring branch between the feed nozzle and the measuring member for the purpose of delivering an electrical signal. The pressure read by the pressure sensor is representative of the distance between the object to be measured and the measuring member. Unfortunately, the pressure sensed by the pressure sensor also depends on the feed pressure from the gas source, such that variations in said pressure spoil the measurement. To compensation for variations in the pressure of the gas source, devices have been made that further include a reference branch comprising an admission nozzle and a calibrated leak, a differential pressure sensor being connected to the measurement branch and to the reference branch.
The advantage of performing the measurement by means of a differential pressure sensor disposed between a reference branch and a measurement branch is that the consequences of variations in the feed pressure are reduced. Measurement is performed initially by calibrating the apparatus, i.e. by adjusting both the zero of the associated electronic device and the leakage section of the reference branch so that the pressure difference is zero when a standard piece having dimensions exactly equal to the looked-for dimensions in pieces to be checked is placed facing the measurement member. When a piece to be checked is inserted in the measurement device the pressure difference measured by the differential pressure sensor is proportional to the dimensional difference between the piece to be checked and the standard piece.
When the feed pressure of the device varies, the pressures in the measurement branch and in the reference branch change in the same direction so that their difference provides some compensation for the variations in the feed pressure. However, this compensation is partial only, and the relative error obtained on the dimensional difference is proportional to the relative variation in the absolute pressure of the feed. The effectiveness of the compensation thus diminishes as the measurement pressure departs from the initial pressure in the reference branch. In practice, that means that the greater the error in the piece to be checked relative to the required dimension, the more the measurement is sensitive to differences in feed pressure. This constitutes a major drawback since it is precisely when close to the acceptable limit tolerance on a piece that it is desirable to have the greatest accuracy in determining whether the piece can be retained or must be rejected.
It is also known that the voltage delivered at the output of the pressure sensor is proportional to the feed pressure of the device. Proposals have thus been made to divide the output signal from the measurement transducer electronically by a signal representative of the absolute feed pressure by generating said signal with a pressure sensor associated with a pressure gauge measuring the absolute feed pressure at the inlet to the device. However, electronically dividing one signal by another signal is an operation that is difficult to perform and that is of doubtful reliability.
It has also been observed that the output voltage of the pressure sensor is proportional to the power supply voltage applied to the pressure sensor itself. An ideal solution for making the measurement independent of the feed pressure would thus consist in keeping constant the product of the power supply voltage to the pressure sensor multiplied by the feed pressure to the measurement device. This equation is represented by a curve that is a hyperbola on a graph where the power supply voltage to the pressure sensor is plotted relative to the feed pressure to the device, and it could be satisfied by making the power supply voltage to the pressure sensor vary in a manner that is inversely proportional to the variation in the feed pressure to the measurement device. This would nevertheless be equivalent to performing electronic division with the problems outlined above.
An object of the present invention is thus to correct variations in feed pressure by replacing the above-mentioned hyperbola with an approximate curve that is easily implemented electronically.